Most highways and public roads have a crown (surface is higher in the middle), so that water will naturally run to the sides and into storm drains.
Purpose-built race tracks generally do not have crowns (flat surface = better grip), so whether or not puddles will form depends on just how flat the surface actually is, on how different areas of the track are graded (pitched) to direct water flow, on the placement of dedicated drainage systems, and on the ability of the surrounding land to absorb water.
Even tracks that are very well built and have generally good drainage tend to have some standing water in places - e.g. small rivulets where water runs across the track as the slope of the track or the surrounding land changes, or areas of the track that have settled slightly over time and collect water. Generally, these are dealt with effectively by wet-weather tires, but depending on the depth of the water, speed of the car (not to mention the tread depth and overall effectiveness of the tread pattern), they may have a significant effect on the car - ranging from feeling a slight “pull” on the steering wheel, to complete hydroplaning and loss of grip.
I don’t know that Forza’s physics are detailed enough to be able to factor in things like tread depth and tire wear when determining whether a tire can successfully deal with standing water without hydroplaning. At the very least, it should be able to take speed into account, as the faster you go, the less time the tire has to disperse the water. A few mm of standing water at 50mph… probably not a big deal. At 150mph, it’s much more treacherous.
Personally, I feel like wet-weather driving in FM23 needs some kind of standing water to be present. I agree that extreme weather like heavy thunderstorms are kind of irrelevant to a track racing game, as no one would actually race in those conditions.
But there definitely should be instances where we can feel a twitch or a slight tug on our wheels from the drag of the tire passing through the water. Maybe not to the degree that it was depicted in FM6, which to me felt a little extreme, but something. As things stand currently, it feels like rain is basically just an overall grip debuff - you can still take the same lines (in addition to avoiding standing water, you should need to alter your line to avoid heavily rubbered-in sections, as they’ll become more slippery).